1. Field of the Invention
The present invention relates to an optical information recording medium and more particularly relates to an optical information recording medium that is additionally recordable by heat mode.
2. Description of the Related Art
Heretofore, an optical information recording medium (optical disc) capable of one time-only recording by laser light is known. This optical disc is also called an additionally recordable CD (so-called CD-R). A typical construction of the optical disc comprises a transparent discoid substrate having thereon a recording layer comprising an organic dye, a reflective layer comprising a metal such as gold, and a protective layer (cover layer) comprising a resin, in the order listed. Recording of information on the CD-R is performed by irradiation with laser light in a near-infrared region (usually laser light having a wavelength of about 780 nm). The irradiated portion of the recording layer absorbs the light and the temperature rises locally. The rise in the temperature causes a change in a physical or chemical property of the irradiated portion (such as pit formation) and thus brings about a change in optical properties of the irradiated portion. As a result, the information is recorded, on the other hand, reading (reproduction) of the information is also carried out by irradiation of the CD-R with laser light having the same wavelength as that of the laser light for recording and is based on detection of difference in reflectance between a portion of the recording layer (recorded portion) whose optical properties have changed and a portion of the recording layer (unrecorded portion) whose optical properties have not changed.
In recent years, there is a demand for an optical information recording medium that enables information recording in a higher density. In order to meet such a demand, an optical disc, which is called an additionally recordable digital versatile disk (so-called DVD-R), has been proposed (for example, “Nikkei New Media” extra issue “DVD” in 1995). The DVD-R comprises two disks, each including a transparent discoid substrate which has pregrooves formed thereon pregrooves for tracking laser light for irradiation having a narrow groove width (0.74 to 0.8 μm), i.e., equal to or less than half of that for CD-R, and further has disposed thereon a recording layer usually containing an organic dye, a reflective layer, and a protective layer, laminated in the order listed. The disks are adhered to each other such that the recording layers face inwardly. Alternatively, the DVD-R comprise a disk having the above-described construction and a discoid protective substrate of the same shape, in which the disk and the discoid protective substrate are adhered to each other such that the recording layer faces inwardly. In this DVD-R, recording and reproduction of information are performed by irradiating visible laser light (usually a laser in a wavelength region ranging from 630 to 680 nm). Accordingly, higher density information recording is possible in comparison with CD-R.
Recently, networks, such as the Internet, and high-vision TV are rapidly coming into wide use. In addition, televising of HDTV (High Definition Television) will start soon. In such a situation, a need exists for a large-capacity recording medium that enables recording of image information in an inexpensive and simple way. Although DVD-R presently fills the role of a large-capacity recording medium, in view of the increasing demand for larger capacity and higher density, the development of a recording medium capable of fulfilling such demand is also necessary. Because of this, the development of a larger-capacity recording medium, which enables recording at a higher density with light having a shorter wavelength than that used in DVD-R, is under way.
For example, Japanese Patent Application Laid-Open (JP-A) Nos.4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818, and 2000-228028 disclose recording and reproducing methods, in which recording and reproduction of information are carried out in an optical information recording medium, which has a recording layer containing an organic dye, by irradiating the recording medium with laser light having a wavelength of 530 nm or less from a recording layer side thereof toward a reflective layer side thereof. According to these methods, recording and reproduction of information are carried out by irradiating an optical disc, which has a recording layer containing a compound, such as porphyrin compound, an azo-based dye, a metal azo-based dye, a quinophthalone-based dye, a trimethinecyanine dye, a dicyanovinylphenyl-skeletal dye, a coumarin compound, a naphthalocyanine compound, or the like, with laser light having a blue color (wavelength: 430 nm, 488 nm) or with laser light having a blue-green color (wavelength: 515 nm).
Meanwhile, an optical disc based on phase transition is known among DVD's. According to this DVD, an alloy layer such as GeSbTe is used as a recording layer, and the recording layer is instantly heated by laser light to cause a phase transition from a crystalline state to an amorphous state so that recording and reproduction are carried out utilizing a change in reflectance changeable in accordance with the phase transition. Recently, a DVD system, in which recording and reproduction are carried out utilizing this phase-transition type DVD by a blue-purple laser, was made public (“ISOM2000” pp. 210 to 211). The task of achieving a high density was fulfilled to some extent by this system.
The optical information recording media described above have been associated with a in problem that, if the recording layer comprises an organic substance and a cover layer is to be adhered using an adhesive, the adhesive dissolved the organic substance constituting the recording layer. Although this problem can be solved by interposing an interlayer between the recording layer and the cover layer, a new problem emerges in that, if a thickness of the interlayer is large, the interlayer brings about a reduction in reflectance.